Geometric effects control isothermal oxidation of graphite flakes

Abstract:

The oxidation, in a neat oxygen atmosphere, of high-purity and highly crystalline natural
graphite and synthetic Kish graphite was investigated. The physico-geometric model function
of the kinetic rate equation was experimentally determined by isothermal thermogravimetric
analysis at 650 °C. Analytic solutions for basic flake shapes indicate that this function strictly
decreases with conversion. However, for both samples the experimental data trend was a
rapid initial increase followed by the expected decrease to zero. High resolution field
emission scanning electron microscopy (FEGSEM), of partially oxidized flakes, provided
plausible explanations for this discrepancy. Rapid development of macroscopic surface
roughness during the initial stages of oxidation was evident and could be attributed to the
presence of catalytic impurities. Large fissures along the planes of the natural graphite and
the initiation, growth and coalescence of internal cavities in the Kish graphite were observed.
Flake models incorporating the latter two features are difficult to analyse analytically.
However, a facile probabilistic approach showed that reasonably good agreement with
experimental data was possible.